What is HSQL database

I was listening about HSQL database, HyperSQLDB etc these days. What are these. Are they are same as regulaar RDBMS databases like oracle, DB2. How do we handle them within the hibernate applications. I mean does using HSQL database different from oracle etc RADBMS databases from hibernate perspective?. How to get started with them.what are the plugins, downloads, configurations i need to have to work with them. Any ideas, suggestions, links, resources, sample code highly appreciated. thanks in advance.